The manufacture of many types of work pieces requires the substantial planarization of at least one surface of the work piece. Examples of such work pieces that require a planar surface include semiconductor wafers, optical blanks, memory disks, and the like. Without loss of generality, but for ease of description and understanding, the following description of the invention will focus on applications to only one specific type of work piece, namely a semiconductor wafer. The invention, however, is not to be interpreted as being applicable only to semiconductor wafers.
One commonly used technique for planarizing the surface of a work piece is the chemical mechanical planarization (CMP) process. In the CMP process a work piece, held by a work piece carrier, is pressed against a polishing surface in the presence of a polishing slurry, and relative motion (rotational, orbital, linear, or a combination of these) between the work piece and the polishing surface is initiated. The mechanical abrasion of the work piece surface combined with the chemical interaction of the slurry with the material on the work piece surface ideally produces a planar surface.
The construction of the carrier and the relative motion between the polishing pad and the carrier head have been extensively engineered in an attempt to achieve a uniform removal of metal across the surface of the work piece and hence to achieve the desired planar surface. However, as a CMP process proceeds, conditions or parameters of the process may change, slightly or dramatically, causing the process to “drift” away from providing a uniform work piece surface. In addition, the CMP process may change from work piece to work piece, or the work pieces themselves may vary enough from each other that achieving uniform removal or a uniform work piece surface on each work piece becomes difficult.
Accordingly, it is desirable to provide a method for monitoring the removal of a metal layer from work piece during a CMP process. It also is desirable to provide a method that generates a signal when the CMP process “drifts”, either during the planarization of a work piece or from work piece to work piece to prevent misprocessed wafers. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.